1. Field of the Invention
This invention relates to a fuel cell power generation system and, more particularly, to a solid-state polymer fuel cell power generation system for an electric vehicle.
2. Description of the Related Art
From the viewpoint of environmental concerns, electric vehicles are preferred because they are driven by clean electric power.
Fuel cells employing hydrogen gas and oxygen gas as fuel have been used as a source of electric power for driving electric vehicles; however, since a fuel cell alone cannot provide a sufficient driving range, a hybrid electric power unit has been developed combining a fuel cell, having large energy capacity and small output capacity, and a rechargeable battery having small energy capacity and large output capacity.
Such a hybrid electric power unit is disclosed in Japanese Patent Application Laid-open No. Hei 3-276573. This prior art system inputs a signal indicative of degree of accelerator opening through a potentiometer to an operation system, determines in the operating system an amount of fuel gas to be fed to the fuel cell based on a load command corresponding to the degree of accelerator opening, and controls the feed of fuel gas according to that determination. As a result, by changing the amount of the fuel gas fed to the fuel cell in response to the change of the driving load, the output of the fuel cell can be changed within, for example, a range of 3 kW, whereby the vehicle receives drive power corresponding to the degree of accelerator opening.
In the above-described conventional system, since the hydrogen gas supplied from a hydrogen cylinder is used as the fuel gas, it has been required that the hydrogen cylinder be carried in the vehicle, along with the electric power unit, with the result that the total weight of the vehicle is increased and maintenance involving changing the cylinders becomes complex. Furthermore, since oxygen gas is used as an oxidizing agent, there exists the danger of explosion in the vehicle.
When a fuel cell generation system is applied to an electric vehicle, it is advisable that the fuel cell generation system be structured to use a hydrocarbon type liquid fuel instead of a hydrogen cylinder, to produce a hydrogen-rich reformed gas by steam reformation reaction of the hydrocarbon type liquid fuel and to feed the reformed gas to the hydrogen electrode in the fuel cell. It is most desirable that a solid-state polymer fuel cell having good energy efficiency and small size be used as the fuel cell.
However, the working temperature of a solid-state polymer fuel cell is almost 100.degree. C., and in the low-temperature region, the electrode core catalyst (Pt) becomes poisoned by carbon monoxide in the reformed gas, with the result that the performance of the fuel cell becomes unstable and short-lived. As a possible solution, the carbon monoxide concentration of about 1% included in the reformed gas just after the reformation reaction could be decreased to less than 100 ppm, more preferably less than 10 ppm, before the reformed gas is fed to the hydrogen electrode.
Conventionally, the reformed gas undergoes a two-step CO removal process, in which the reformed gas is first subjected to a CO shift reaction (CO+H.sub.2 O.fwdarw.H.sub.2 +CO.sub.2) by a shifting catalyst so as to decrease the carbon monoxide concentration to about 1,000 ppm, and the thus treated gas is then passed through a CO remover for CO removal (CO+1/20.sub.2 .fwdarw.CO.sub.2) by a selective-oxidation catalyst (Pt/Al.sub.2 O.sub.3), so as to decrease the carbon monoxide concentration to 100 ppm or less.
In the above-described conventional system, only the fuel gas feed rate is controlled responsive to changes of load on the vehicle, so that the space velocity in the CO remover deviates from the desired range and, therefore, it is difficult to satisfactorily decrease the carbon monoxide concentration in the reformed gas.